I've looked on this site and found no threads specifically regarding frame differences.I have also looked through several old (1990s) frame books from Car-O-Liner and Tru-Way.
Here are some of the things I think I know:
I know there are different frame lengths due to body, or cab & bed configurations, also refered to as the wheelbase.
I know there are different frames for Broncos and trucks, due to the rear frame on Broncos being narrower, or "pinched", and Broncos have the shortest frame, body, or wheelbase.
I know there is one frame that is used for 2wd & 4wd coil spring trucks, and a different frame is used for 4wd leaf spring trucks.
I know the engine cross member is different depending on which engine it holds or used originally.
I know there are different suspensions, mounts, and crossmembers used depending on the weight capacity rating of the vehicle (ie: Bronco, 150, 250, 350).
I did not want to get into F-Super Duties or cab & chassis as I believe they are a different configuration altogether.
BUT what I do not know is if there is any difference in the guage (or thickness) of the frame or the actual strength of the material used to manufacture each different type (as described above) of frame.
Since, for the most part, all the sheetmetal is interchangeable (some modifications for truck cab to Bronco body & vice versa) I would say the frame rails are the same.
Lastly, I believe that the capacity of the truck is determined by the suspension it uses and not the frame.
Anyone else, please chime in. If it is your opinion or experience or a fact please state that as well and where it comes from or why. Thanking all in advance for additional information or correcting my incorrect information.

 

1/2 tons and broncos are thinner material than 3/4 and 1 tons

Also the year plays a part as well in the thickness

 

I don't know where the cutoff was but I suspect it was the same 8500lb level that affected other aspects. The frame for trucks above that were bigger in every way than their LD cousins, the channel is taller and the material itself is a heavier gauge.

 

When you say "the engine cross member is different depending on which engine it holds or used originally"

Are you just referring to the things that the 302/351 mounts to that aren't present on the other crossmembers?

Are there bolt holes in different places?

Are the crossmembers dimensionally different?

While the first two might warrant a separate part number I would only call the third a "different" crossmember.

 

The 302/351 engine cross member has a hole for the oil filter and the 460 (and I believe diesels) have more of a notch out of the front of the x-member.
I don't know about the 300, though I'd guess it's the same as the small blocks.
The motor mount towers are rivet/bolted on and engine specific.

 

I remember seeing different part numbers for the engine cross member for 6 cylinder, small block, big block, and diesel. This was over 15 years ago, and scribbled in the frame book, when we had to do a complete frame swap on an insurance job at the shop I used to work at. So my notes showed 4 engine cross members, not the part numbers as apparently we were able to buy a complete frame from a local dealer for the truck, unlike a Suburban frame we did that came with out any brackets or crossmembers that differed.
I am always running project ideas through my head and always wonder what the REAL differences in the frames are. Friends are always asking me about ideas too. So I'm just trying to be better informed.
Currently a friend has a 1950s F100 that he wants to put on a FREE 1980s Bronco frame. I don't think it will work due to 5 inch difference in wheel base, but hey anything is possible with time, treasure, and talent.
Thanks all who replied. Any more?

 

With later models (sorry, don't know years) watch out for the LD F250s with the semi float rear end. Same frame as F150 at 3/16" thick compared to HD frames at 1/4" thick.

 

The 300 cross-member in my 94 F150 and the 351 cross-member I cut out of a 95 F250 are identical (both 4wd) once you drill the rivets out of the SBF mounts and remove the TTB pivot brackets. The F250 was rusted completely out at the frame flanges starting about at the transmission cross-member so I have no idea what it's original thickness. Best I could tell it was the the exact same as my F150 (except for the mounts for front leafs and, lack of crumple zones and extra length where the extended cab was).